1. Field of the Invention
The present invention relates to an ignition circuit for a squib in an air bag in a vehicle, especially to an ignition circuit for a squib which can boost the energy supplied to the squib.
2. Description of the Related Art
Recently, vehicles generally contain air bag systems to protect drivers and passengers. In the air bag system, a squib is ignited by a current supplied from a battery when a collision is detected by the sensor arranged behind the head light of the vehicle. Gaseous nitrogen is generated by the squib, and an air bag which is installed in the steering wheel of the vehicle is quickly inflated.
As the air bag is required to function without fail when a collision is detected, it is recommended that an air bag system contain a backup capacitor storing sufficient energy to ignite the squib if the battery voltage should fall.
It is, however, required that a current above a threshold level is continuously supplied for a predetermined threshold period. Therefore, the energy stored in the backup capacitor (hereinafter referred to as the capacitor) must be more than a threshold amount. It is, therefore, required that the capacity of the capacitor must be larger, or the voltage applied to the capacitor must be higher.
However, more space is required to mount a large capacitor on the vehicle, or a DC-DC converter for increasing the battery voltage (hereinafter referred to as the converter) is required.
The addition of a converter to the air bag system, however, makes the system expensive, because the converter is expensive, and the noise generated by the converter affects other electronic circuits.